Any image containing exactly one copyrighted or original character. No one else should be visible in the image.
Although the metatag chartags:1 can be used to find most such cases, solo can be used to distinguish a single unnamed or cosplaying character (among other things).
These situations should not be tagged solo:
- Single characters being depicted from multiple views (post #12904)
- dakimakura which contain multiple views
- dual persona
- time paradox
These situations should be tagged solo focus, not solo:
- Pictures focusing on one character among a crowd of nondescript, less detailed people.
- Single characters engaged in sex acts with faceless males or faceless females.
source: danbooru wiki